1. Field of the Invention
This invention relates generally to surgical instruments and more particularly to a thermoelectric cryosurgery tool and a method of using the same.
2. Description of the Prior Art
Cryosurgery, which relates to the destruction of living tissue by freezing cells, is commonly used to destroy benign and malignant tumors or neoplasms of the skin and mucous membranes. For example, cryosurgery is a commonly used treatment for warts, sun damage, keratoses, liver spots, and basal cell carcinoma.
Modified cryosurgery has also been used to treat inflammatory diseases of the skin, including acne. Cryotherapy has been used in nerve disorders such as the management of chronic pain and the heart disfunction AV-Node Reentrant tachycardia. Cryosurgery has also been effective in the ocular fields for open angle glaucoma treatment, cataract, and lens removal.
Cryosurgery is preferred to scalpel surgery for the treatment of many lesions because it is quick, rather painless, and often does not require local anesthesia, has a very low risk of infection post-operatively, and generally leaves a more cosmetically acceptable scar, if any. Cryosurgery is preferred to electrosurgery for the treatment of these lesions because the scar is cosmetically more acceptable and the depth of tissue destruction is greater.
The mechanisms, identified in the literature, causing cell destruction during cryosurgery, are several. First, if the cells are cooled slowly water leaves the cell resulting in high concentrations of toxic electrolytes which necritize. Second, if the rate of freeze is carried out too quickly, the ice crystals formed are small and less damage producing. During the thaw, if the "ice to water" phase change occurs too quickly, all ice simply melts. If thaw rates are slow, the ice crystals elongate as smaller crystals melt and refreeze to their neighbors, causing cell wall rupture. Lastly, cell death occurs when metabolism continues while ice blocks cell nutrient supply and waste disposal.
Cryosurgery may be performed through the direct application of refrigerants, such as dry ice, liquid nitrogen, nitrous oxide, or chlorofluorocarbons (CFC's) to the tissue at the treatment site. Methods of application include direct spray to the tissue, dipping a cotton swab into the liquid cryogen and then applying the soaked swab to the site or spraying the cryogen into a closed tip metal tube which is in contact with the site. However these techniques are undesirable for several reasons. First, the direct application of cryogens, dip or spray, causes a very fast freeze, resulting in the creation of tiny ice crystals instead of the large damage producing type. The cells also do not have time to dehydrate, reducing the damage caused by high electrolyte concentrations. After the cryogen is removed the heat from inside the body causes a relatively fast thaw, reducing the further elongation of ice crystals during the "ice to water" phase change, which occurs during a slower thaw rate. The application of spray into a closed tube gives some temperature rate control, but with great difficulty and imprecise cycle reproducibility. Second, some methods use CFC's which are hazardous to the environment, while others use nitrous oxide which can be toxic to the patient and physician. Thirdly, the literature states that many times the above treatments are not 100% effective due to the inability to induce enough of the mechanisms of destruction and the patient must return for successive treatments at later dates.
Fourth, the technique requires the purchase and storage of expensive, volatile refrigerants which quickly evaporate, no matter how well they are insulated. In addition, the direct use of refrigerants may expose the patient to the potential risk of cryoinjury resulting from refrigerant run-off or over-spray. Contamination of the refrigerant supply has also proved to be a drawback. Due to the expense, evaporation, and storage difficulties, general practitioners, small clinics, hospitals, military field hospitals, and underdeveloped countries do not normally keep cryogenic materials on hand, but refer their patients to a specialist, which causes greater inconvenience and cost to the patient.
It has been proposed to confine the refrigerant within a hollow cryoprobe. A cryoprobe typically includes a metal tip and a thermally insulated handle for holding a supply of the refrigerant. However, these probes also suffer from the storage and evaporation problems associated with the direct application of the refrigerants.
Beginning in approximately the 1960's, thermoelectric cryoprobes based on the Peltier effect of thermoelectric cooling were introduced. These probes utilized thermoelectric heat pumps, constructed from junctions of opposing P-type and N-type semiconductors. When an electric current is passed through the junction, the resulting electron flow pumps heat from the cold junction to the hot junction of the heat pump. A cascade of such devices, each cooling the hot junction of its neighbor, can reach cryogenic temperatures electrically, eliminating the need to store and apply dry or liquid refrigerants.
Examples of thermoelectric cryoprobes includes U.S. Pat. No. 3,502,080/Re26,276 (Hirschhorn), U.S. Pat. No. 3,369,549 (Armao) and U.S. Pat. No. 4,519,389 (Gudkin et al.).
Gudkin et al. discloses a semiconductor thermoelectric element mounted on a handle.
The Hirschhorn reference discloses a hand-held surgical instrument having a cutting edge or tool connected to a metal rod. The metal rod in turn is in direct physical contact with a plurality of Peltier elements, allowing the cutting edge or tool to be cooled considerably below ambient room temperature.
The Armao reference discloses a probe containing several thermoelectric elements capable of delivering either cryogenic or thermal temperatures for the treatment of tissue.
Typically, prior art devices have relied on solid metal thermal conductors and have operated the thermoelectric modules in parallel to increase the heat flow to the desired level. Configurations of this type are usually limited to a maximum temperature differential on the order of 65.degree. C. The use of solid conductors also degrades performance because of the high temperature gradients supported by solid metallic conductors. A one-inch long copper or silver conductor is not suitable for cryosurgery, even if side losses are ignored, because of its high thermal resistance.
A heat pipe is a heat transfer device, usually tubular in shape, which is completely self contained, and has no moving mechanical parts. In general, cryogenic heat pipes are vacuum insulated. Cryogenic pipes have a condenser end which is cooled, causing the gas to condense. The condensed liquid is absorbed by a wick and flows by capillary action to the evaporator end. As heat is applied to the evaporator end, some of the liquid evaporates. This gas travels through the hollow center at near sonic speeds to the cooled, condenser end where it gives up heat, recondenses and starts the cycle again through the wick.
Heat pipes are useful since they assume a nearly isothermal condition while transporting large quantities of heat. Thus, heat pipes can transfer several hundred times the amount of heat that is transferred by metallic conductors at the same temperature drop. A properly designed heat pipe requires as little as one thousandth the temperature differential needed by a copper rod to transmit a given amount of power between two points.
In addition, heat pipes have the ability to regulate heat-flux transformation. As long as the total heat flow is in equilibrium, the fluid streams connecting the condenser and evaporator ends essentially are unaffected by the local heat flux in these regions. In fact, heat pipes which are two feet long lose only a couple of degrees from one end of the pipe to the other. A high heat flux input at one point of the evaporator end can be coupled with a large area of lower heat flux in the condenser end.
The direct application of liquid refrigerants is very effective because the physical evaporation of cryogens achieves a great flux density. The vaporization of cryogens in a heat pipe evaporator works the same without loss to the atmosphere.
Heat pipe models featuring flow through a closed end tube are nearly as effective as the above-described cryoprobe, with only a small thermal resistance at the end of the tube.
Cascades of Peltier devices are also known in the art. For example, An Instrument Using a Multiple Layer Peltier Device to Change Skin Temperature Rapidly, published in Brain Research Bulletin, Vol. 12, 1984, teaches a three layer Peltier device utilized in a pain research application. The device incorporates proportional feed back control to provide stable temperatures within the treated tissue.
None of the prior art Peltier instruments, including Frigitronics, are able to achieve temperatures lower than -25.degree. to -35.degree. C.